Tunneling Magnetoresistance (tmr) In Magnetic Tunnel Junctions

Because of the widespread actual applicability and the theoretic research value, electronic transportation nature has always been the hotspot of scientific research in a long time. With the advancement of science and technology, the size of electronic device has been ceaselessly reduced. Therefore, the study on electronic transportation nature has been turned into the mesoscopic field. Since the discovery of the giant magnetoresistance effect (GMR) in magnetic multilayers and magnetic granular systems and the discovery of tunneling magnetoresistance effect (TMR) in ferromagnetic tunnel junctions, the spin-polarized transport of electrons has become one of the important regions of condensed matter physics due to their potential applicability and abundant physics. In this dissertation, we study the TMR effect in ferromagnetic tunnel junctions.In the first two chapters, we first introduced the significance and survey of the magnetic tunnel junctions system. Then the correlative mechanism of the TMR effect on spin- polarization and tunneling phenomenon etc. is briefly generalized. Following, we analyzed the two familiar theoretical explanations of Julliere model and Slonczewski model, and analyzed the tunneling Hamiltonian method and quantum-mechanical tunneling method, and then compared the two theoretical methods.In this dissertation, we study theoretically the spin-polarized transport and TMR effect in the ferromagnet/ semiconductor/ ferromagnet and ferromagnet/ half-metal/ ferromagnet tunnel junctions. The main content is described as follows:In Chapter 3, considering the Rashba spin-orbit interaction in the semiconductor, we study theoretically the spin-polarized transport in a two-dimensional ferromagnetic semiconductor double tunnel junctions with a quantum-mechanical approach. It is found that the transmission coefficient shows typical resonant transmission properties and the Rashba spin-orbit coupling has great different influences on the transmission coefficients of electrons with spin-up and down and tunneling magnetoresistance (TMR). We find that T_P↑↑ has no oscillation with the variation of K R/K 0 and a regardless of Z, however, T_{AP↑↓(↓↑)} and T_P↓↓ does have. Furthermore, as the thickness of the SM layer a increases, the TP↓↓and TA P_{↑↓(↓↑)} oscillates more rapidly, the peaks become sharper, and the intervals between adjacent peaks are narrowed. More importantly, the TMR is significantly enhanced by increasing the spin-orbit coupling, which is very useful for the designing of magnetic digital and memory sensor.In Chapter 4, applying the half-metal magnetic material to the ferromagnetic tunneling junction, we study theoretically the resonant tunneling and the TMR effect in a two-dimensional ferromagnetic/ half-metal/ ferromagnetic tunnel junction by a scatter approach, viz. quantum-mechanical approach. The results show that as we choice the spin-up subband in the half-metal material shows a metal property, the transmission coefficients of electrons with spin-up and down show typical resonant transmission properties. With the difference of the split energy between spin-up and spin-down subband in half-metal increases, T_P↓↓ and T_AP oscillate more rapidly, the intervals between adjacent peaks are narrowed, however, T_P↑↑ is nearly not affected by the change of split energy in the half-metal, holding the intrinsic oscillatory period. Furthermore, the difference of split energy in half-metal has great different influences on the TMR and the relative difference of the conductorΔG/G|-. They are significantly enhanced by increasing the difference of the split energy between the spin-up and spin-down subband in half-metal. It is obvious that the half-metal material is very useful for enhancing the TMR, so we can obtain the perfect results by choice appropriate parameters. Thereby, it is useful for improve the capability of magnetism organ such as magnetism memorizer etc.